Well, NaiveDateTime is just a struct / map after all:

iex(1)> NaiveDateTime.now() |> Map.keys()
[:__struct__, :calendar, :day, :hour, :microsecond, :minute, :month, :second,
 :year]

The calendar field is usually the module/atom Calendar.ISO so you can strip it if you don’t deal with other calendars, and the special __struct__ field, and then end up with this:

iex(2)> NaiveDateTime.utc_now() |> Map.from_struct() |> Map.drop([:__struct__, :calendar])
%{
  day: 10,
  hour: 12,
  microsecond: {981284, 6},
  minute: 32,
  month: 3,
  second: 40,
  year: 2021
}

You can then also remove the decimal precision value from microsecond by using elem(..., 0) and then you’ll just have seven unsigned 32-bit integers (which you can encode with int32 since it uses variable-length encoding over the wire). That resulting map of seven integers can then very easily be described as a nested Protobuf structure afterwards.

Of course, you can also convert the NaiveDateTime to UNIX seconds / milliseconds / microseconds / nanoseconds and just encode the whole thing as a 64-bit unsigned integer:

naive_date_time_from_another_function
|> DateTime.from_naive!("Etc/UTC") # assuming your times are in UTC
|> DateTime.to_unix(:microsecond) # or :second, or :millisecond, or :nanosecond)